For example, in order to feed numerous calves at one time, there may be a row of teats projecting horizontally around the top perimeter of a container and those teats would normally be connected to tubes equipped with one-way valves to enable liquid to be drawn up from the bottom of the container when the teat is sucked. Obviously in such a construction, as long as the teat level is above the liquid level, there is no danger of appreciable quantities of the liquid leaking out of the teat under gravitational pressure.
However, where the teats are to be gravity fed they may be situated, for example, at the base of a container so that there is a head of liquid above the teats of perhaps up to 2 metres, and it then becomes imperative that the teats be self-sealing and able to resist that amount of pressure.